This study presents distortion analysis of the fully differential Gm-C filters in which the imbalance of the differential input voltages is taken into consideration. Closed-form equations expressing distortions of differential-mode (DM) output voltages as functions of DM and common-mode input voltages are developed. It was found that the HD3 of the DM output voltage is independent of the input imbalance. On the contrary, the HD2 of the DM output voltage is directly dependent upon the input imbalance. Simulation results are shown to be in good agreement with the analytical results.